1. Field of the Invention
The present invention relates to a method of recovering characteristics of a deteriorated cation exchange membrane used in an electrolysis of an alkali metal chloride. More particularly, it relates to a method of recovering characteristic of a deteriorated cation exchange membrane used in an electrolysis of an aqueous solution of an alkali metal chloride to obtain chlorine and an alkali metal hydroxide.
2. Description of the Prior Arts
It has been known to produce chlorine in an anode compartment and to produce an alkali metal hydroxide in a cathode compartment by partitioning the anode compartment and the cathode compartment in the cell with a diaphragm and supplying an aqueous solution of an alkali metal chloride.
An asbestos diaphragm has been used as the diaphragm. However, in the asbestos diaphragm method, the alkali metal hydroxide is contaminated with an alkali metal chloride to decrease the purity and the alkali metal hydroxide having high concentration could not be obtained advantageously. In order to overcome the disadvantages, the use of a cation exchange membrane as the diaphragm has been proposed and considered as the important process.
The disadvantages of the asbestos diaphragm process could be overcome, however, when the cation exchange membrane is continuously used, the current efficiency of the membrane is gradually decreased and a cell voltage is gradually increased to be uneconomical.
On the other hand, the cation exchange membrane for electrolysis of an alkali metal chloride is usually expensive so that most of the cost for producing an alkali metal hydroxide is spent as the cost for the membrane.
From these viewpoints, it is economically advantageous to recover characteristics of the deteriorated membrane or to use repeatedly the membrane.
It has been proposed to recover characteristics of a cation exchange membrane from these viewpoints, for example, a method of swelling a perfluorocarbon type cation exchange membrane with an organic solvent and removing the organic solvent from the membrane (Japanese Unexamined Patent Publication No. 130,491/1977); a method of heat-treating a fluorinated carboxylic acid type cation exchange membrane (Japanese Unexamined Patent Publication No. 3999/1978); a method of contacting a perfluorocarbon type cation exchange membrane with a solution of an acid having oxidizing property (Japanese Unexamined Patent Publication No. 37598/1978). Various other methods have been proposed to improve the recovery of cation exchange membranes.
The inventors have studied to improve the recovery of characteristics of the deteriorated cation exchange membrane for electrolysis of an alkali metal chloride.